The present invention relates generally to the field of voice processing, and more particularly, to narrowband voice communication systems. Narrowband analog systems generally utilize analog voice transmitters with a limited transmission channel bandwidth (telephones, AM radios, CB and HAM radios, etc.). Narrowband digital systems generally take the form of digitally encoded voice transmitters with a limited front-end bandwidth (voice coders, often called VOCODERS).
The transmission bandwidth of analog voice systems is typically 3 kilo-Hertz (kHz). The front-end bandwidth in digital voice systems is typically about 4 kHz. This is a reasonable compromise between the requirements for intelligible speech and the reduction of the data rate. However, while a bandwidth of three to four kHz is adequate for the reproduction of intelligible vowels, it is not high enough for most of the voiceless fricatives, whose frequency spectra are concentrated above 4 kHz. Examples of voiceless fricative or sibilant sounds are /s/, /sh/ and /ch/. FIG. 1A illustrates the frequency response when certain words are pronounced, some with fricative sounds. It can be seen that a greater portion of the fricative spectra energy is above 4 kHz than below.
The use of a broader front-end bandwidth would be a simple solution for digital voice systems, but would lead to an undesirable increase in the data rate. Likewise, the use of transmission media having a broader bandwidth would be a simple solution for analog voice systems, but such a solution is often not available. In analog voice systems, the problem has been solved in part, by the use of a carbon microphone, which spreads the frication noise throughout the spectrum by the random modulation of the electric resistance caused by the movement of the carbon granules. However, the rest of the speech signal from the carbon microphone is distorted as well.